This invention relates to an improvement in the structure of a keyboard switch that functions as an input device for a point of sale ("POS") terminal, an electronic cash register ("ECR"), or the like.
A typical conventional keyboard switch in such a device comprises a single piece that combines a keyboard housing and a key stem guide sleeve. A key stem fits into the key stem guide sleeve. Depressing a keytop causes the key stem to slide downward. A keytop support joins the keytop to the key stem. The tip of a coil spring set inside the key stem becomes a movable contact that connects stationary contacts on a printed wiring board ("PWB").
Such a keyboard switch is vulnerable to the infiltration of dust or water through the gap between the inner surface of the key stem guide sleeve and the outer surface of the key stem. To eliminate this drawback, the present inventor proposed a novel keyboard switch (see Japanese Utility Model Application No. 74274/'91) in which a keytop support connects the keytop and the key stem. Two annular grooves, the first cut in the undersurface of the keytop support, the second cut around the key stem guide sleeve (which is integral with the keyboard housing), hold an elastic hollow cylinder under compression, thereby preventing dust or water from infiltrating into the key stem guide sleeve between the inner surface of the sleeve and the outer surface of the key stem.
This earlier-invented switch, however, still has a problem. Dust or water cannot be completely prevented from passing into the key stem guide sleeve through the gap between the key stem and the attaching portion of the keytop support, since the key stem and the keytop support are not integral. Rather they are separate parts that must be put together.